The identification of the presence or absence of specific nucleic acid sequences in a sample is a central part of many assays and tests used in the modern research lab and clinical setting. In the typical scheme, the nucleic acids from the sample are first separated from other macromolecules present in the sample by manipulating various physical properties. For example, nucleic acids typically bear a net negative charge at neutral pH, owing to the phosphodiester backbone. This property can be manipulated to separate nucleic acids from other macromolecules using anion exchange resins. As another example, differential solubility of nucleic acids compared to other macromolecules in certain solvents is used to extract nucleic acids from the sample. Numerous other such schemes exist. However, the amount of target nucleic acid relative to the total amount of nucleic acid purified typically is very low. Therefore, some type of amplification is necessary. Either the amount of specific nucleotide sequence(s) is increased by target amplification methods such as polymerase chain reaction (PCR) or the specific nucleotide sequence(s) is/are reacted with a detectable label and the signal from the label is amplified to detectable levels.
Unfortunately, these methods have limited utility. One limitation is that target-specific amplification methods such as PCR are inherently error-prone. For example, although the stringency of primer hybridization can be controlled, there nonetheless exists the potential for non-specific primer binding and primer-independent amplification, which can lead to false-positive results. Moreover, different sequences can amplify at different rates, resulting in amplification bias. As a result, quantitative analysis of multiple nucleic acid sequences in a single reaction often suffers from a lack of sensitivity. In addition, target nucleic acids that are present at low concentrations relative to other nucleic acids may be effectively “masked” from the polymerase, which could result in false-negative results. Other factors may exist that reduce both the specificity and sensitivity of such assays. Another limitation of PCR is that a relatively small fragment of the target is amplified. As a result, in case of mutations/deletions the assay may produce false-negative results.
Therefore, methods and compositions are needed for specific and sensitive isolation and analysis of at least one target nucleic acid segment containing at least one specific sequence.